As a part of the investigation on the structure of vessel walls in hardwoods, the structure of the primary wall was studied using ultra-thin sectioning and replica techniques. Microfibrils of the primary wall extend straight and are arranged parallel one another. The primary wall is composed of three layers, P_O, P_M and P_I, each showing the different microfibrillar orientation. Microfibrils of the outer layer P_O are oriented transversely with respect to the vessel axis and microfibrils of the middle layer P_M are oriented at random. The inner layer P_I consists of the crossed polylamellate structure. On the other hand, microfibrils of the primary wall pass through the pit membrane maintaining their orientation. The crossed polylamellate structure is also present in the pit membrane. From these facts, it is considered that the structure of the pit membrane is essentially same as that of the primary wall.